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Abstract:

The present invention relates to a process of preparing a nanogap
electrode and a nanogap device using the same, and a preparing process
according to the present invention is characterized in that reduced metal
is grown by reduction reaction from a metal ion in solution on the
surface of a metal pattern with a predetermined shape. A method of
preparing a nanogap electrode according to the present invention has an
advantage that nanogap electrodes having a gap distance of 1-100 nm,
which are difficult to prepare by a conventional method, can be easily
prepared in a reproducible and uniform manner.

Claims:

1. A method of preparing a nanogap electrode, wherein reduced metal is
grown by reduction reaction from a metal ion in solution on the surface
of a metal pattern with a predetermined shape.

2. The method of preparing a nanogap electrode according to claim 1,
wherein the metal pattern is formed by any one method selected from
electron beam lithography, photo lithography, X-ray lithography, and
printing method.

3. The method of preparing a nanogap electrode according to claim 1,
wherein the metal pattern and a metal which is grown on the metal pattern
are same.

4. The method of preparing a nanogap electrode according to claim 1,
wherein the solution contains water or a mixed solvent of water and an
organic solvent.

5. The method of preparing a nanogap electrode according to claim 4,
wherein the concentration of a metal ion in the solution is 1 μM-1 mM.

6. The method of preparing a nanogap electrode according to claim 1,
wherein the gap distance of the formed nanogap electrode is 1-100 nm.

7. The method of preparing a nanogap electrode according to claim 1,
wherein the metal pattern is selected from Au, Ag, Al, Cu, and Pt.

8. The method of preparing a nanogap electrode according to claim 1,
wherein a substrate formed with a metal pattern having a predetermined
shape is immersed in a solution containing a metal ion, and then reduced
metal is grown from the metal ion in solution on the surface of the metal
pattern by adding a reducing agent to the solution.

9. The method of preparing a nanogap electrode according to claim 8,
wherein the metal ion is reduced by adding a reducing agent selected from
hydroxylamine (H2NOH), ascorbic acid, glucose, Rochelle salt,
formaldehyde, and their mixture to the solution.

10. The method of preparing a nanogap electrode according to claim 1,
wherein the metal ion is selected from HAuCl4, AgNO3, AuCl,
AuCl2, AuCl3, AuCl4, Au(CO)Cl, NaAuCl4, and their
mixture.

11. A nanogap electrode, wherein the electrode is prepared by a preparing
method according to claim 1.

12. The method of preparing a nanogap electrode according to claim 6,
wherein a substrate formed with a metal pattern having a predetermined
shape is immersed in a solution containing a metal ion, and then reduced
metal is grown from the metal ion in solution on the surface of the metal
pattern by adding a reducing agent to the solution.

13. The method of preparing a nanogap electrode according to claim 6,
wherein the metal ion is selected from HAuCl4, AgNO3, AuCl,
AuCl2, AuCl3, AuCl4, Au(CO)Cl, NaAuCl4, and their
mixture.

Description:

TECHNICAL FIELD

[0001]The present invention relates to a process of preparing a nanogap
metal electrode and a nanogap device using the same, and a preparing
process according to the present invention is characterized in that
reduced metal is grown by reduction reaction from a metal ion in solution
on the surface of a metal pattern formed with a predetermined shape.

BACKGROUND ART

[0002]The present invention relates to a process of forming an electrode
having a nanogap. A nanogap electrode means an electrode having a gap
distance of about 1-100 nm. As a process of preparing a nanogap electrode
comes out in recent years, new technology has been developed at a fast
speed in the field of measuring and applying the characteristics of
nano-tube, nano-particle, nano-wire, or the like as well as materials
having the size of nanometer scale, such as protein and DNA.

[0003]However, it is very difficult to prepare the nanogap of 100 nm or
less due to the restriction of its process when the conventional
semiconductor process technologies are used.

[0004]In recent years, there has been proposed a method of forming a
nanogap or angstrom (Å) gap by means of mechanical break junction (C.
Zhou, C. J. Muller, M. R. Deshpande, J. W. Sleight, and M. A. Reed, Appl.
Phys. Lett. 67, 1160 (1995); R. Reichert, R. Ochs, D. Beckmann, H. B.
Weber, M. Mayor, and H. v. Lohneysen, Phys. Rev. Lett. 88, 176804-1
(2002)), electromigration (H. Park, A. K. L. Lim, A. P. Alivisatos, J.
Park, and P. L. McEuen, Appl. Phys. Lett. 75, 301 (1999)) and the like,
but these methods are only useful for a method of forming a very narrow
gap of about 1 nm, but difficult to prepare a nanogap having the range of
3-100 nm. Moreover, these methods are not easy to be commercialized due
to its low reproducibility, and it is impossible to prepare an
arbitrary-shaped nanogap or multiple nanogaps.

[0005]On the other hand, a method of forming a nanogap electrode on a
semiconductor substrate by wet-etching of a mesa structure is publicly
known (R. Krahne, A. Yacoby, H. Shtrikman, I. Bar-Joseph, T. Dadosh, and
J. Sperling, Appl. Phys. Lett. 81, 730 (2002)), but it also cannot be an
economical and reproducible method for preparing a nanogap electrode
having an arbitrary shape or multiple nanogap electrodes, which are
separated by several nanometers.

[0006]Besides, there is a method of electrodeposition (C. Z. Li, H. X. He,
and N. J. Tao, Appl. Phys. Lett. 77, 3995 (2000); A. F. Morpurgo, C. M.
Marcus, and D. B. Robinson, Appl. Phys. Lett. 74, 2084 (1999)) or the
like, but this method is also complicated in its preparing process, and
has a disadvantage that a nanogap electrode having an arbitrary shape or
multiple nanogap electrodes cannot be prepared in the same manner as
break junction technology or the like.

DISCLOSURE

Technical Problem

[0007]The present invention is devised in order to solve the
above-mentioned problems, and an object of the present invention is to
provide a method of preparing a reproducible nanogap electrode, and
another object of the invention is to provide a method of preparing a
nanogap electrode having an arbitrary shape or multiple nanogap
electrodes, as well as to provide an economical method of preparing a
nanogap electrode which can be used in the field of biosensor or the
like, furthermore, to provide a device using a nanogap electrode which is
prepared through this method.

Technical Solution

[0008]The present invention relates to a process of preparing a nanogap
electrode and a nanogap device using the same. A preparing process
according to the present invention is characterized in that reduced metal
is grown by reduction reaction from a metal ion in solution on the
surface of a metal pattern formed with a predetermined shape. The method
of growing metal on the surface of a metal pattern is surface-catalyzed
chemical deposition. According to the present invention, interdigitated
nanogap electrodes having a gap distance of 1-100 nm can be prepared with
high yield (greater than 90%) and reproducibility. A nanogap electrode of
about 10 nm can be prepared, which is particularly difficult to be
prepared.

[0009]A method of preparing a nanogap electrode is specifically
characterized in that a substrate formed with a metal pattern having a
predetermined shape is immersed in a solution containing a metal ion, and
then reduced metal is grown from the metal ion in solution on the surface
of a metal pattern by adding a reducing agent. Furthermore, metal can be
grown by repeating a step of immersing a substrate with a metal pattern
in several reaction baths filled with a solution containing a metal ion
and a reducing agent, where the concentration of metal ion can be same
for reaction baths, or the metal ion solution having a different
concentration can be used.

[0010]A method of preparing a nanogap electrode according to the present
invention will be described with reference to FIG. 1. Using a metal
pattern 2 formed with a predetermined shape, a metal growth layer 3 is
formed by the reduction reaction of a metal ion on the metal pattern 2.
The gap of the metal pattern 2 is not particularly restricted but it is
appropriate to have a gap of about 50-500 nm, and the metal pattern 2 can
be formed on a substrate 1 by a typical method selected from electron
beam lithography, photo lithography, X-ray lithography, printing method,
and the like, and the metal may be Au, Ag, Al, Cu, or Pt.

[0011]The metal pattern may further include a metal adhesive layer which
is selected from Ti, Ni, Cr, or the like, between the substrate and the
metal pattern to increase the adhesion to the substrate.

[0012]For the metal ion, considering the common use with the metal
pattern, it is preferable to use an ion which is resulted from the same
metal as the metal pattern, but it may be also possible to use a
different metal ion, and in fact any metal ion can be employed if it has
conductivity through reduction. The metal ion is exemplified by
HAuCl4, AgNO3, AuCl, AuCl2, AuCl3, AuCl4,
Au(CO)Cl, NaAuCl4, and CuSO4, and water or a mixed solvent of
water and an organic solvent can be used as a solvent for dissolving the
metal ion, and it is preferable to have low concentration of about 1
μM-1 mM.

[0013]On the other hand, a substrate formed with a metal pattern is
immersed in a solution containing the metal ion, and then the metal ion
in the solution is reduced by a reducing agent, thereby being deposited
and grown on the metal pattern to form a nanogap. For the reducing agent,
it is preferable to use a weak reducing agent for appropriately
controlling the reduction rate of metal ions, including Lewis acid or
weak Bronsted acid, and especially it can be exemplified by hydroxylamine
(H2NOH), ascorbic acid, glucose, Rochelle salt, formaldehyde.

[0014]The method of preparing a nanogap electrode according to the present
invention is characterized in that the condition of weak reducing agent
and low metal ion concentration is employed, thereby removing the
possibility of nucleation in solution and metal being grown selectively
only on the surface of a metal pattern where the surface energy is high.

[0015]The reaction formula occurring on the surface of a metal pattern is
shown as follows, for example, in case of hydroxylamine (H2NOH).

H3NOH+H3NOHads+-,

H3NOHads+H2NOads+2H++e.sup.

H2NOads→HNOads+H++e.sup.

HNOadsNOads+H++e.sup.

AuCl4-+3e-Au0+4Cl.sup.

[0016]A reducing agent H2NOH is protonated, and then absorbed ("ads"
means an absorbed state on the metal surface) to be oxidized to NO on the
surface of a metal pattern, and metal ions are deposited onto the metal
pattern by reduction reaction on the location where the NO is absorbed.

[0017]In the process of forming a nanogap according to the present
invention, a growth rate distribution is illustrated by arrows in FIG. 1.
As metal growth is progressed, the mass transfer is hindered in the gap
region between the metal electrodes, thereby the reaction speed (growth
rate) being gradually slowed down. The gap distance of a nanogap
electrode can be controlled by the concentration of metal ions and
reducing agent and their reaction time, and it will become narrow as the
reaction time and concentration increase.

DESCRIPTION OF DRAWINGS

[0018]The above and other objects, features and advantages of the present
invention will become apparent from the following description of
preferred embodiments given in conjunction with the accompanying
drawings, in which:

[0019]FIG. 1 is a schematic diagram illustrating a process of preparing a
nanogap electrode;

[0020]FIG. 2 is a FESEM photograph (a) of a gold pattern formed by an
electron beam lithography method, and a FESEM photograph (b) of a nanogap
gold electrode formed on the surface of the gold pattern;

[0021]FIG. 3 is an I-V graph of a nanogap electrode as illustrated in FIG.
2(b);

[0022]FIG. 4 is a FESEM photograph of a nanogap electrode formed with a
three-pole electrode according to the present invention;

[0023]FIG. 5 is a FESEM photograph of an interdigitated gold electrode;

[0024]FIG. 6 is a FESEM photograph of an interdigitated nanogap gold
electrode formed on the surface of the interdigitated gold electrode as
illustrated in FIG. 5;

[0025]FIG. 7 is a FESEM photograph of 20,000 nanogap electrodes; and

[0026]FIG. 8 is a FESEM photograph (a) of 40×10 nanogap electrodes,
and a histogram (b) of the gap distance of the nanogap electrodes.

DETAILED DESCRIPTION OF MAIN ELEMENTS

[0027]1: substrate, 2: metal pattern, 3: metal growth layer

Best Mode

[0028]Hereinafter, the embodiments of the present invention will be
described in detail with reference to accompanying drawings. A method of
preparing a nanogap electrode according to the present invention will be
described through the embodiments, but those embodiments should not be
construed to restrict the scope of the invention.

Embodiment 1

[0029]An electron resist (ER) was coated on a SiOx substrate to form an
electron resist pattern using an electron beam lithography process, and
then 10 nm Ti was deposited as a metal adhesive layer and 50 nm Au was
then deposited, and then a Ti/Au (10 nm/50 nm) having a gap distance of
about 40 nm was made using a typical electron beam lithography process
for lifting off the electron resist. Subsequently, it was dipped in a
piranha solution (H2SO4/30% H2O2=5:1(v/v)) at
50° C. for 10 minutes, and then washed with pure water several
times and then dried in nitrogen atmosphere.

[0030]1 mL of 400 μM HAuCl4 aqueous solution was added to 10 mL of
water in which a substrate formed with the gold pattern is immersed, and
then 1 mL of 640 μM NH2OH aqueous solution was added, and then
reacted at 27.5° C. for 2 minutes, and this process was repeated
four times.

[0031]FIG. 2 is a FESEM photograph (a) showing a gold pattern formed by an
electron beam lithography, and a FESEM photograph (b) showing a nanogap
gold electrode formed on the surface of the gold pattern. When the FESEM
photograph of a gold pattern of FIG. 2(a) prior to metal growth is
compared with the FESEM photograph (b) of a nanogap gold electrode formed
on the surface of the gold pattern according to the embodiment 1 of FIG.
2(a), it is confirmed that the gap distance of about 40 nm prior to gold
growth became narrowed to about 1 nm.

[Experimental Example] Measurement of a Nanogap

[0032]Based on an equation expressed in a paper written by P. Steinmann et
al. (J. Vac. Sci. Technol. B 22, 3178(2004)) and a measured value of the
nanogap electrode prepared by the embodiment 1, it is confirmed that the
nanogap distance was 1 nm.

[0033]In case where the gap of a pattern becomes very narrow to 2 nm or
less as shown in FIG. 2(b), it may be difficult to measure a nanogap
using SEM. Then, the gap distance can be estimated using an electrical
measurement value and a mathematical calculation, which is expressed as
Equation 1.

[0034]An I-V curve expressed by a solid line in FIG. 3 is obtained by a
least-square method for the measured values, and the I-V curve is
obtained with s=1.0 nm, φ=0.8 eV, and A=3.0×10-15 cm2
in the Equation 1. This matches well with a paper written by Hahn et al.
(Appl. Phys, A:Mater. Sci. Process. 66, S467(1998)) where the φ value
(barrier height) is described to be about 0.8 eV. As a result, it is
confirmed that the nanogap distance of a nanogap electrode of the
embodiment 1 was 1 nm.

Embodiment 2

[0035]Except for forming a Ti/Au pattern into three-poles on a SiOx
substrate, this was similarly progressed as described in embodiment 1,
and in FIG. 4 it is illustrated a FESEM photograph of a nanogap electrode
with three-poles.

Embodiment 3

[0036]An interdigitated gold pattern having a gap of about 100 nm and a
length of 15 μm on a SiOx substrate was similarly made by an electron
beam lithography process as described in embodiment 1, and then a process
was repeated four times, wherein a substrate formed with the gold pattern
was placed into 11 mL of 36 μM concentrated HAuCl4 aqueous
solution, and 1 mL of 640 μM NH2OH aqueous solution was added,
and then reacted at 27° C. for 2 minutes, thereby allowing gold to
be grown on the surface of the interdigitated gold pattern to prepare
interdigitated nanogap gold electrodes.

[0037]FIG. 5 is a FESEM photograph of a interdigitated gold pattern used
in the embodiment 3, and FIG. 6 is a FESEM photograph of interdigitated
nanogap gold electrodes formed on the surface of the interdigitated gold
pattern. Nanogap electrodes maintaining a uniform width of about 30 nm as
illustrated in FIG. 6 was formed after gold was grown on the surface of
the gold pattern having a gap distance of about 100 nm as illustrated in
FIG. 5.

[0038]FIG. 7 is a FESEM photograph of 20,000 nanogap electrodes formed
within a 1 mm×1 mm area, which is fabricated by a method similar to
those of the embodiments, and it is confirmed that these electrodes were
prepared with a gap distance of about 2 nm.

[0039]FIG. 8(a) is a FESEM photograph of 40×10 nanogap electrodes,
and FIG. 8(b) is a histogram of gap distances before or after preparing
the nanogap electrodes as illustrated in FIG. 8(a). It is confirmed that
the gap having average distance of 42±7.6 nm became narrowed to have
average distance of 3.3±1.4 nm. It is also confirmed that very uniform
nanogap can be prepared by a method according to the present invention,
from the fact that the standard deviation of the gap distance was ±7.6
nm in the case of a metal pattern whereas it was reduced to ±1.4 nm
after a metal layer was grown.

INDUSTRIAL APPLICABILITY

[0040]A method of preparing a nanogap electrode according to the present
invention has advantages that nanogap electrodes can be easily prepared
in a reproducible and uniform manner by controlling the concentration of
a reactant and reaction time, and that the interdigitated nanogap
electrodes applicable to the field of biosensor or the like can be
prepared in an economical manner.